Device and Method for Tuning Threshold Voltage by Implementing Different Work Function Metals in Different Segments of a Gate

ABSTRACT

A semiconductor device includes an active region spanning along a first direction. The semiconductor device includes a first elongated gate spanning along a second direction substantially perpendicular to the first direction. The first elongated gate includes a first portion that is disposed over the active region and a second portion that is not disposed over the active region. The first portion and the second portion include different materials. The semiconductor device includes a second elongated gate spanning along the second direction and separated from the first elongated gate in the first direction. The second elongated gate includes a third portion that is disposed over the active region and a fourth portion that is not disposed over the active region. The third portion and the fourth portion include different materials.

PRIORITY DATA

This application is a continuation of U.S. patent application Ser. No.15/624,402, filed Jun. 15, 2017, which claims priority from U.S.Provisional Patent Application No. 62/490,248, entitled “Device andMethod for Tuning Threshold Voltage by Implementing Different WorkFunction Metals in Different Segments of a Gate” and filed on Apr. 26,2017, the disclosures of which are incorporated herein in theirentirety.

BACKGROUND

The semiconductor industry has progressed into nanometer technologyprocess nodes in pursuit of higher device density, higher performance,and lower costs. As this progression takes place, challenges from bothfabrication and design issues have resulted in the development ofthree-dimensional designs, such as fin-like field effect transistor(FinFET) device. A typical FinFET device is fabricated with a thin “fin”(or fin-like structure) extending from a substrate. The fin usuallyincludes silicon and forms the body of the transistor device. Thechannel of the transistor is formed in this vertical fin. A gate isprovided over (e.g., wrapping around) the fin. This type of gate allowsgreater control of the channel. Other advantages of FinFET devicesinclude reduced short channel effect and higher current flow.

FinFET devices are compatible with a high-k metal gate (HKMG) processflow. In other words, FinFET devices may be implemented as HKMG devicesthat have a high-k gate dielectric and a metal gate electrode. However,existing HKMG FinFET devices still have shortcomings, for exampleshortcomings related to lack of threshold voltage (Vt) tuning options,which limits the IC circuit design freedom and could also degrade deviceperformance.

Therefore, although existing HKMG FinFET devices have been generallyadequate for their intended purposes, they have not been entirelysatisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion. It is also emphasized that thedrawings appended illustrate only typical embodiments of this inventionand are therefore not to be considered limiting in scope, for theinvention may apply equally well to other embodiments.

FIG. 1 is a perspective view of an example FinFET device.

FIG. 2 illustrates a top view of a FinFET device according to anembodiment of the present disclosure.

FIG. 3 is a graph illustrating a relationship between leakage and drivecurrent for a transistor according to an embodiment of the presentdisclosure.

FIG. 4 is a graph illustrating a relationship between threshold voltageand a distance according to an embodiment of the present disclosure.

FIG. 5 illustrates a circuit schematic for an SRAM cell according to anembodiment of the present disclosure.

FIG. 6 is a flowchart illustrating a method according to an embodimentof the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The present disclosure is directed to, but not otherwise limited to, afin-like field-effect transistor (FinFET) device. The FinFET device, forexample, may be a complementary metal-oxide-semiconductor (CMOS) deviceincluding a P-type metal-oxide-semiconductor (PMOS) FinFET device and anN-type metal-oxide-semiconductor (NMOS) FinFET device. The followingdisclosure will continue with one or more FinFET examples to illustratevarious embodiments of the present disclosure. It is understood,however, that the application should not be limited to a particular typeof device, except as specifically claimed.

The use of FinFET devices has been gaining popularity in thesemiconductor industry. Referring to FIG. 1, a perspective view of anexample FinFET device 50 is illustrated. The FinFET device 50 is anon-planar multi-gate transistor that is built over a substrate (such asa bulk substrate). A thin silicon-containing “fin-like” structure(hereinafter referred to as a “fin”) forms the body of the FinFET device50. The fin extends along an X-direction shown in FIG. 1. The fin has afin width W_(fin) measured along a Y-direction that is orthogonal to theX-direction. A gate 60 of the FinFET device 50 wraps around this fin,for example around the top surface and the opposing sidewall surfaces ofthe fin. Thus, a portion of the gate 60 is located over the fin in aZ-direction that is orthogonal to both the X-direction and theY-direction.

L_(G) denotes a length (or width, depending on the perspective) of thegate 60 measured in the X-direction. The gate 60 may include a gateelectrode component 60A and a gate dielectric component 60B. The gatedielectric 60B has a thickness t_(ox) measured in the Y-direction. Aportion of the gate 60 is located over a dielectric isolation structuresuch as shallow trench isolation (STI). A source 70 and a drain 80 ofthe FinFET device 50 are formed in extensions of the fin on oppositesides of the gate 60. A portion of the fin being wrapped around by thegate 60 serves as a channel of the FinFET device 50. The effectivechannel length of the FinFET device 50 is determined by the dimensionsof the fin.

FinFET devices offer several advantages over traditional Metal-OxideSemiconductor Field Effect Transistor (MOSFET) devices (also referred toas planar transistor devices). These advantages may include better chiparea efficiency, improved carrier mobility, and fabrication processingthat is compatible with the fabrication processing of planar devices.FinFET devices are compatible with a high-k metal gate (HKMG) processflow. Thus, FinFET devices may be implemented as HKMG devices where thegates each that have a high-k gate dielectric and a metal gateelectrode. For these benefits discussed above, it may be desirable todesign an integrated circuit (IC) chip using HKMG FinFET devices for aportion of, or the entire IC chip.

However, traditional HKMG FinFET devices may still have shortcomings,for example shortcomings with respect to threshold voltage (Vt) tuning.In more detail, for many modern devices (e.g., cell phones, computers,etc.), the ability to fine-tune the threshold voltage may be desirablein order to optimize a performance-power tradeoff. The threshold voltageof a MOSFET transistor (including FinFETs) is largely determined by thematerial composition of a work function metal inside the gate electrodeof the transistor. One way of adjusting the threshold voltage is toincrease the gate length of the transistor. However, as semiconductorfeature sizes continue to shrink, increasing the gate length may not bea realistic option. Ion implantation may also be used to adjust thethreshold voltage. However, such ion implantation may cause damage tothe FinFET device and may degrade the device performance. Due to theselimitations, IC designers often have to make compromises with respect tothreshold voltage tuning, which means that the IC design is often notoptimized, for example with respect to a performance-power tradeoff.

According to the various aspects of the present disclosure, differentwork function metals are implemented in different segments of the gatestructures formed outside an active region and over the active region,respectively. By configuring the distance between the active region andthe gate segments (having different work function metals) formed outsidethe active region, and/or by choosing the specific material compositionsof these work function metals, the present disclosure offers thecapability to tune the threshold voltage with more granularity. In otherwords, the present disclosure allows the threshold voltage for FinFETdevices to be more finely tuned, as discussed in more detail below.

FIG. 2 is a diagrammatic fragmentary top view of a semiconductor device100 according to embodiments of the present disclosure. In someembodiments, the semiconductor device 100 includes a FinFET device suchas the FinFET device 50 of FIG. 1. The semiconductor device 100 includesan active region 110, also referred to as an OD region. In someembodiments, the active region 110 includes a fin structure (e.g.,similar to the fin structure shown in FIG. 1) that extends in theX-direction. The fin structure may include a semiconductor material,such as silicon or silicon germanium. Source/drain regions 120 (similarto the source 70 and drain 80 of FIG. 1) are also formed in the finstructure. The source/drain regions 120 may be formed by processes suchas ion implantation. A dielectric isolation structure 130 (e.g., ashallow trench isolation) surrounds (or is disposed adjacent to) theactive region 110. For example, a boundary 140 (extending in theX-direction) of the active region 110 separates the active region 110from the dielectric isolation structure 130.

The semiconductor device 100 also includes a plurality of elongated gatestructures that each extends in the Y-direction (perpendicular to theX-direction), for example elongated gate structures 150, 151, 152, and153. In some embodiments, the elongated gate structures 150-153 are HKMGstructures and each includes a high-k gate dielectric and a metal gateelectrode. A high-k dielectric material is a material having adielectric constant that is greater than a dielectric constant of SiO2,which is approximately 4. In an embodiment, the high-k gate dielectricincludes hafnium oxide (HfO2), which has a dielectric constant that isin a range from approximately 18 to approximately 40. In alternativeembodiments, the high-k gate dielectric may include ZrO2, Y2O3, La2O5,Gd2O5, TiO2, Ta2O5, HfErO, HfLaO, HfYO, HfGdO, HfAlO, HfZrO, HfTiO,HfTaO, or SrTiO.

The metal gate electrode may include a work-function layer and afill-metal layer. The work functional metal component is configured totune a work function of its corresponding transistor to achieve adesired threshold voltage Vt. The work function metal component mayinclude at least one layer, or a plurality of layers formed of differentmaterials. The material of the work-function layer is selected accordingto whether the respective FinFET is an n-type FinFET or a p-type FinFET.For example, when the FinFET is an n-type FinFET, the work-functionlayer may include a TaN layer and a titanium aluminum (TiAl) layer overthe TaN layer and a TiAl layer. When the FinFET is a p-type FinFET, thework-function layer may include a TaN layer, a TiN layer over the TaNlayer, and a TiAl layer over the TiN layer. Other suitable materials forthe work-function layer may include titanium aluminum nitride (TiAlN),tantalum carbon nitride (TaCN), titanium nitride (TiN), tungsten nitride(WN), tungsten (W), or combinations thereof. The fill-metal layer isformed over the work-function layer and serves as the main conductiveportion of the elongated gate structures 150-153. In variousembodiments, the fill-metal layer may include aluminum (Al), tungsten(W), copper (Cu), or combinations thereof.

In some embodiments, the formation of the elongated gate structures150-153 involves a gate-replacement process. In more detail, a pluralityof dummy gate structures may be formed first, which may include a dummygate dielectric such as silicon oxide and a dummy gate electrode such aspolysilicon. After the formation of the source/drain regions 120, thedummy gate structures are removed and replaced by the elongate gatestructures 150-153 that each contain a high-k gate dielectric and ametal (or conductive) gate electrode. In some embodiments, thegate-replacement process involves replacing both the dummy gatedielectric and the dummy gate electrode. In other embodiments, thehigh-k gate dielectric is formed below the dummy gate electrode, andonly the dummy gate electrode needs to be replaced by the metal gateelectrode as a part of the gate-replacement process.

As shown in FIG. 2, the elongated gate structures 150-153 each have arespective segment 150A-153A that is disposed over the active region110. The elongated gate structures 150-153 also each have a respectivesegment (or portion) 150B-153B that is not disposed over the activeregion 110. In other words, the segments 150B-153B of the elongated gatestructures 150-153 are disposed outside of the active region 110 and aredisposed over the dielectric isolation structures 130.

According to various aspects of the present disclosure, at least some ofthe segments disposed not over the active region 110 are configured tohave a different material composition than the segments that aredisposed over the active region 110. For example, the segment 151B andthe segment 151A have different material compositions, the segment 152Band the segment 152A have different material compositions, and thesegment 153B and the segment 153A have different material compositions.

In some embodiments, at least some of the segments 150A-153A have thesame material compositions as one another, but the segments 151B-153Bmay have the same or different material compositions as one another. Forexample, in some embodiments, the segments 150A-153A may each have afirst material composition, and the segments 151B-153B may each have asecond material composition that is different from the first materialcomposition. In some other embodiments, the segments 150A-153A may eachhave a first material composition, the segment 151B may have a secondmaterial composition, the segment 152B may have a third materialcomposition, and the segment 153B may have a fourth materialcomposition. The second, third, and fourth material compositions may bethe same (but still different from the first material composition) insome embodiments, or they may be different from one another in otherembodiments.

The different material compositions of the segments 151B-153B (e.g.,different from the segments 151A-153A) helps tune the threshold voltageVt, as discussed in more detail below. In some embodiments, therespective material compositions of the segments 151B-153B may beconfigured by one or more metal deposition processes performed as a partof the gate-replacement process (e.g., when the gate structures 151-153are formed).

Still referring to FIG. 2, the segments 151B-153B are spaced apart fromthe segments 151A-153A. For example, the segment 151B is separated fromthe boundary 140 of the active region 110 by a distance 171, the segment152B is separated from the boundary 140 of the active region 110 by adistance 172, and the segment 153B is separated from the boundary 140 ofthe active region 110 by a distance 173, where the distances 171-173 areall measured in the Y-direction.

In some embodiments, at least some of the distances 171, 172, and 173are different from one another. For example, the distances 171 and 172may be substantially equal, but the distance 173 may be larger than (orsmaller than) the distance 171 or the distance 172. As another example,the distance 172 may be larger than the distance 171, while the distance173 may be larger than the distance 171 and the distance 172. In someembodiments, the difference between any of the distances 171/172/173 andthe rest of the distances 171/172/173 may be greater than 10% of any ofthe distances 171/172/173. For example, the distances 171 and 172 mayeach be equal to M nanometers (nm), and the distance 173 is greater thanthe distances 171 and 172, and the distance 173 may be from about 15 nmto an allowed distance depending on how much Vt shift the designer wantsto achieve.

Still referring to FIG. 2, the segments 151B-153B may have differentsizes or dimensions as well. For example, the segment 151B may have adimension 181, the segment 152B may have a dimension 182, and thesegment 153B may have a dimension 183, where the dimensions 181-183 areeach measured in the Y-direction.

In some embodiments, the dimensions 181-183 may be substantially equalto one another. In other embodiments, however, at least some of thedimensions 181-183 are different from the other dimensions 181-183. Forexample, the dimension 183 may be smaller than (or larger than) thedimension 181 or the dimension 182 in some embodiments. In someembodiments, the difference between any of the dimensions 181/182/183and the rest of the dimensions 181/182/183 may be greater than 10% ofany of the dimensions 181/182/183. For example, if the dimension 181 and182 are each equal to N nm, and the dimension 183 is greater than thedimensions 181 and 182, then the dimension 183 is greater than 1.1×N nm,where N is greater than about 26 nm. It is understood that thedimensions 181/182/183 are correlated with the distances 171/172/173.For example, as the dimensions 181/182/183 increase, the distances171/172/173 may decrease, respectively, and still achieve the desired Vtadjustment. Vice versa, as the distances 171/172/173 become larger, thedimensions 181/182/183 may become smaller, while still achieving thedesired Vt adjustment. In this manner, it can be seen that thedimensions 181/182/183 and the distances 171/172/173 are actuallyinterdependent.

One novel aspect of the present disclosure is that it allows thethreshold voltage Vt to be flexibly tuned by implementing the segments151B-153B that have different material compositions than the segments151A-153A. As discussed above, the segments 151B-153B are portions ofthe elongated gate structures that are not located over the activeregion 110, whereas the segments 151A-153A are portions of the elongatedgate structures that are located over the active region 110. Since thethreshold voltage Vt of a given transistor is dependent on the materialcompositions of the gate electrode (e.g., the material composition ofthe work-function layer), the different material compositions of thesegments 151B-153B have an impact on the overall Vt of eachcorresponding transistor. In other words, by configuring the materialcompositions of each of the segments 151B-153B, the correspondingtransistor's threshold voltage may be adjusted accordingly (either up ordown).

In addition, the threshold voltage may be further tuned by configuringthe distances 171-173 that separate the segments 151B-153B from theboundary 140 of the active region 110, via a Metal Boundary Effect(MBE). For example, as the distance 171/172/173 decreases, the influenceexerted by the segments 151B/152B/153B on their respective transistor'sVt may increase. Conversely, as the distance 171/172/173 increases, theinfluence exerted by the segments 151B/152B/153B on their respectivetransistor's Vt may decrease. As such, the threshold voltage Vt of acorresponding transistor may be further tuned by configuring the valueof the distance 171/172/173. In some embodiments, the value of thedistance 171/172/173 may be set by using a logic operation (LOP)computer-aided design (CAD) layer as a part of IC design/layout.

Furthermore, the threshold voltage may also be tuned by configuring thedimensions 181-183 of the segments 151B-153B. For example, as thedimension 181/182/183 decreases, the influence exerted by the segments151B/152B/153B on their respective transistor's Vt may decrease as well.Conversely, as the dimension 181/182/183 increases, the influenceexerted by the segments 151B/152B/153B on their respective transistor'sVt may increase as well. As such, the threshold voltage Vt of acorresponding transistor may be further tuned by configuring the valueof the dimension 181/182/183. In some embodiments, the value of thedimension 181/182/183 may be set by using a logic operation (LOP)computer-aided design (CAD) layer as a part of IC design/layout.

Based on the above discussions, it can be seen that the thresholdvoltage Vt of a HKMG FinFET transistor is a function of: a work-functionlayer material composition of the segment of the gate that is not formedover the active region, the distance separating this segment of the gatefrom the boundary of the active region, and/or the size of this segmentof the gate. Note that not all of the gate structures need to have suchas segment. For example, even though the gate structure 150 has asegment 150B that is not disposed over the active region 110, thissegment 150B has the same material composition as the segment 150A thatis disposed over the active region 110.

The Vt tuning flexibility offered by the present disclosure can bevisually illustrated in FIG. 3, which is a graph 200 illustrating arelationship between leakage and drive current for a transistor (e.g., aHKMG FinFET transistor) according to embodiments of the presentdisclosure. The graph 200 includes an X-axis that represents the drivecurrent for the transistor, and a Y-axis that represents the leakage(e.g., source cutoff current (Isoff)) for the transistor. Eachregion/spot in the graph 200 may correspond to a particular thresholdvoltage Vt. In other words, each different threshold voltage isassociated with a corresponding drive current and leakage.

As the drive current for the transistor increases, its performanceimproves. As the leakage for the transistor decreases, its powerconsumption is reduced. Therefore, it is desirable for a transistor tosimultaneously achieve a strong drive current (e.g., moving towards inthe “right” on the X-axis) and a low leakage (e.g., moving “downwards”in the Y-axis). In other words, it is desirable for the transistor toreside near the “bottom right” in the graph 200.

Unfortunately, transistor devices in the real world often has to make atradeoff between the drive current and the leakage, such that as thetransistor's drive current performance improves, its leakage performancedegrades, and vice versa. This tradeoff may be referred to as aperformance-power tradeoff. It may be beneficial for an IC designer tohave different transistors that have different corresponding Vtprofiles, so that the IC designer can choose the appropriate transistor(with its corresponding Vt profile) that is most suitable for thecircuit or IC application in which the transistor is implemented. Forexample, in applications where the device performance is important, theIC designer may wish to use transistors that offer a strong drivecurrent at the expense of a relatively high leakage. Conversely, inapplications where the device standby time is important, the IC designermay wish to use transistors that offer a low leakage at the expense ofreduced drive current.

In order to provide sufficient flexibility in the IC design, asemiconductor manufacturer may offer transistors with differentpredetermined threshold voltages to the IC designer. For example, thesemiconductor manufacturer may offer a standard threshold voltage (SVt)device, a low threshold voltage (LVt) device, and an ultralow thresholdvoltage (ULVt) device. In some embodiments, the SVt device, the LVtdevice, and the ULVt device may be provided by configuring thework-function layer's material composition for the segments of the gateformed over the active region, for example the segments 150A-153A of thegate. The SVt device, LVt device, and ULVt device may be offered to theIC designer as a part of an IC design or layout package/library, wherethe IC designer can freely choose the suitable transistor device to useto implement his/her IC design.

In FIG. 3, the SVt device corresponds to a region 210 in the graph 200,the LVt device corresponds to a region 211 in the graph 200, and theULVt device corresponds to a region 212 in the graph 200. However, giventhe complexity and sophistication of modern day IC applications, havingmerely three different transistors with their respective thresholdvoltages (e.g., SVt, LVt, and ULVt) may be insufficient. An IC designermay need to use transistors with a more versatile threshold voltage Vttuning capability.

As discussed above, the present disclosure allows the threshold voltageVt to be flexibly tuned by implementing a different work-function metalin a portion of the gate that is formed outside (e.g., not disposedabove) the active region. For example, each of the SVt transistordevice, the LVt transistor device, and the ULVt transistor device may beimplemented using the HKMG FinFET transistors shown in FIG. 2. Thesegments 151B, 152B, and 153B of FIG. 2 may each include a differentwork-function metal material than their corresponding segments 151A,152A, and 153A of the gate structures. By carefully configuring thematerial composition of the work-function layers of the segments 151A,152A, and 153A, the threshold voltage Vt of their correspondingtransistor may be adjusted. In FIG. 3, the changing or tuning of thethreshold voltage Vt may be represented as the regions 210/211/212shifting in a direction indicated by the arrows 220/221/222. Forexample, by configuring the material composition of the work-functionmetal material for the segments 151B, 152B, or 153B, the regions 210,211, and/or 212 may move towards the “upper right” direction or towardthe “lower left” direction in the graph 200 of FIG. 3, as pointed by thearrows 220/221/222.

Along similar lines, since the threshold voltage of a transistor mayalso be tuned by configuring the distances 171, 172, or 173 between theboundary 140 of the active region 110 and the segments 151B, 152B, and153B, the regions 210, 211, and/or 212 may be further shifted in thedirection indicated by the arrows 220/221/222 accordingly. Lastly, sincethe dimensions 181, 182, and 183 of the segments 151B, 152B, and 153Bmay also impact the threshold voltage of the corresponding transistor,these dimensions 181, 182, and/or 183 may also be configured tofacilitate the shifting of the regions 210, 211, and/or 212. In thismanner discussed above, regardless of whether a SVt transistor device isused, or a LVt transistor device is used, or an ULVt transistor deviceis used, the corresponding threshold voltage of that transistor devicecan still be flexibly tuned based on the various aspects of the presentdisclosure.

It is understood that the threshold voltage Vt for a given transistormay also be tuned by increasing the gate length, and/or by performingone or more implantation processes to the gate. This aspect of tuningthe threshold voltage Vt may be visually represented in FIG. 3 byshifting the regions 210, 211, and/or 212 in the direction indicated bythe arrows 230, 231, and 232, respectively. The resulting regions 240,241, and 242 may still correspond to a SVt device, a LVt device, and anULVt device, respectively. Of course, the transistor corresponding tothe regions 240, 241, and 242 in the graph 200 may still be tuned in asimilar manner as the transistor corresponding to the regions 210, 211,and 212. For example, the gate structure may include a differentwork-function metal segment in a segment located not over the activeregion 210, and the distance between the active region's boundary andthe different work-function metal segment may be configured, and thedimension of the different work-function metal segment may also beconfigured, in order to flexibly tune the threshold voltage Vt of thetransistors corresponding to the regions 240, 241, and/or 242. As aresult, the regions 240, 241, and/or 242 may be shifted in a directionindicated by the arrows 250, 251, and/or 252, respectively. In thismanner, the present disclosure offers further threshold voltage tuningflexibility compared to conventional devices.

FIG. 4 illustrates a graph 300 that illustrates a relationship ofthreshold voltage versus a distance, where the distance is the distanceseparating the boundary of the active region and the segment of the gatestructure that includes a different work-function metal. For example,the distance may be the distance 171, 172, or 173 of FIG. 2. An X-axisof the graph 300 represents the distance, and a Y-axis of the graph 300represents the threshold voltage Vt of the corresponding transistor.

The graph 300 includes a plot 310 and a plot 311. In some embodiments,the plot 310 represents the threshold voltage of an NFET, and the plot311 represents the threshold voltage of a PFET. In some otherembodiments, the plot 310 represents the threshold voltage of a PFET,and the plot 311 represents the threshold voltage of an NFET. Asillustrated in FIG. 4, the plots 310 and 311 are each a function of thedistance. As the distance (e.g., the distance 171, 172, or 173 of FIG.2) increases, the plot 310 decreases in value (going from a positivenumber toward 0), whereas the plot 311 increases in value (going from anegative number toward 0). Therefore, the graph 300 visually indicatesthat the threshold voltage Vt of a transistor can be tuned byconfiguring the distance 171, 172, or 173 of FIG. 2.

The various aspects of the present disclosure may be useful in many ICapplications. For example, the flexible tuning of the threshold voltagemay be implemented in SRAM cells. As an example, FIG. 5 illustrates acircuit schematic for a SRAM cell 400 for which the threshold voltagetuning according to the present disclosure may be applied. In someembodiments, the SRAM cell 400 may be a 1-bit SRAM cell. The SRAM cell400 includes pull-up transistors PU1, PU2; pull-down transistors PD1,PD2; and pass-gate transistors PG1, PG2. As show in the circuit diagram,transistors PU1 and PU2 are p-type transistors, such as the p-typeFinFETs discussed above, and transistors PG1, PG2, PD1, and PD2 aren-type FinFETs discussed above.

The drains of pull-up transistor PU1 and pull-down transistor PD1 arecoupled together, and the drains of pull-up transistor PU2 and pull-downtransistor PD2 are coupled together. Transistors PU1 and PD1 arecross-coupled with transistors PU2 and PD2 to form a first data latch.The gates of transistors PU2 and PD2 are coupled together and to thedrains of transistors PU1 and PD1 to form a first storage node SN1, andthe gates of transistors PU1 and PD1 are coupled together and to thedrains of transistors PU2 and PD2 to form a complementary first storagenode SNB1. Sources of the pull-up transistors PU1 and PU2 are coupled topower voltage Vcc (also referred to as Vdd), and the sources of thepull-down transistors PD1 and PD2 are coupled to a voltage Vss, whichmay be an electrical ground in some embodiments.

The first storage node SN1 of the first data latch is coupled to bitline BL through pass-gate transistor PG1, and the complementary firststorage node SNB1 is coupled to complementary bit line BLB throughpass-gate transistor PG2. The first storage node N1 and thecomplementary first storage node SNB1 are complementary nodes that areoften at opposite logic levels (logic high or logic low). Gates ofpass-gate transistors PG1 and PG2 are coupled to a word line WL.

FIG. 6 is a flowchart illustrating a method 500 according to anembodiment of the present disclosure. The method 500 includes a step510, in which a fin structure is formed. The fin structure that extendsin a first direction.

The method 500 includes a step 520 of forming a dummy gate structurethat extends in a second direction different from the first direction.

The method 500 includes a step 530 of forming source/drain regions inthe fin structure.

The method 500 includes a step 540 of: after the forming of thesource/drain regions, replacing the dummy gate structure with metal-gatestructure. The metal-gate structure includes a first portion formed overthe fin structure and a second portion not formed over the finstructure. The first portion and the second portion include differentwork-function metals.

In some embodiments, the forming of the dummy gate structure comprisesforming a first dummy gate structure and a second dummy gate structurethat is spaced apart from the first dummy gate structure in the firstdirection. The replacing of the dummy gate structure may be performedsuch that a first metal-gate structure and a second metal-gate structurereplace the first dummy gate structure and the second dummy gatestructure, respectively. The first metal-gate structure may include thefirst portion and the second portion. The second metal-gate structuremay include a third portion formed over the fin structure and a fourthportion not formed over the fin structure. The third portion and thefourth portion may include different work-function metals. In someembodiments, the replacing of the dummy gate structure is performed suchthat: the second portion is separated from the fin structure by a firstdistance measured in the second direction; the fourth portion isseparated from the fin structure by a second distance measured in thesecond direction; and the second distance is greater than or less thanthe first distance. In some embodiments, the replacing of the dummy gatestructure is performed such that: the second portion has a firstdimension measured in the second direction; the fourth portion has asecond dimension measured in the second direction; and the seconddimension is greater than or less than the first dimension.

It is understood that additional processes may be performed before,during, or after the steps 510-540 of the method 500. For example, themethod 500 may include addition steps of forming conductivevias/contacts, interconnect layers, packaging, testing, etc. For reasonsof simplicity, other additional steps are not discussed herein indetail.

Based on the above discussions, it can be seen that the presentdisclosure offers advantages over conventional methods and devices forthreshold voltage tuning. It is understood, however, that otherembodiments may offer additional advantages, and not all advantages arenecessarily disclosed herein, and that no particular advantage isrequired for all embodiments. One advantage is that the presentdisclosure allows for more flexible tuning of a threshold voltage Vt.For example, the threshold voltage for a high-k metal gate (HKMG) FinFETtransistor may be tuned by implementing a segment of the gate structureoutside the active region, such that the segment includes a differentwork-function metal material than portions of the gate formed over theactive region. The threshold voltage may also be tuned by adjusting thedistance between the boundary of the active region and the segment ofthe gate structure formed not over the active region. The thresholdvoltage may be further tuned by configuring the dimension or size ofthat segment of the gate structure. Being able to offer transistors withdifferent threshold voltages is beneficial for the IC designer, as theIC designer may need to access a variety of different transistors withdifferent threshold voltages in order to fine-tune the IC design, forexample fine-tune the IC design to optimize a power-performancetradeoff. This power-performance tradeoff is relevant in many ICapplication areas, such as mobile communication devices. Anotheradvantage is that the present disclosure does not involve makingextensive layout changes and is compatible with existing HKMG FinFETprocess flow. Therefore, the present disclosure is easy and cheap toimplement.

In an embodiment, a semiconductor device includes: an active regionextending in a first direction; and a first gate structure extending ina second direction different from the first direction; wherein: thefirst gate structure includes a first segment that is disposed over theactive region and a second segment that is not disposed over the activeregion; the first segment has a first material composition; and thesecond segment has a second material composition different from thefirst material composition. In some embodiments, the active regionincludes a fin structure. In some embodiments, the first segment of thefirst gate structure at least partially wraps around the fin structure.In some embodiments, the first segment of the first gate structure has afirst metal material composition; and the second segment of the firstgate structure has a second metal material composition different fromthe first metal material composition. In some embodiments, thesemiconductor device further includes a second gate structure extendingin the second direction, the second gate structure being spaced apartfrom the first gate structure in the first direction; wherein: thesecond gate structure includes a third segment that is disposed over theactive region and a fourth segment that is not disposed over the activeregion; the third segment has a third material composition; and thefourth segment has a fourth material composition different from thethird material composition. In some embodiments, the active region has aboundary that extends in the first direction; the second segment isspaced apart from the boundary of the active region by a first distance;and the fourth segment is spaced apart from the boundary of the activeregion by a second distance that is different from the first distance.In some embodiments, the second material composition is different fromthe fourth material composition. In some embodiments, the first materialcomposition and the third material composition are the same. In someembodiments, the second segment has a first dimension measured in thesecond direction; the fourth segment has a second dimension measured inthe second direction; and the first dimension is not equal to the seconddimension.

In an embodiment, a semiconductor device includes: an active regionspanning along a first direction; a first elongated gate spanning alonga second direction substantially perpendicular to the first direction,wherein the first elongated gate includes a first portion that isdisposed over the active region and a second portion that is notdisposed over the active region, and wherein the first portion and thesecond portion include different materials; and a second elongated gatespanning along the second direction and separated from the firstelongated gate in the first direction, wherein the second elongated gateincludes a third portion that is disposed over the active region and afourth portion that is not disposed over the active region, and whereinthe third portion and the fourth portion include different materials. Insome embodiments, the active region includes a fin structure; and thefirst portion and the second portion each wrap around the fin structure.In some embodiments, the active region has a boundary that spans alongthe first direction; the second portion is separated from the boundaryof the active region by a first distance; and the fourth portion isseparated from the boundary of the active region by a second distancethat is greater than or less than the first distance. In someembodiments, the first portion and the second portion include differentconductive materials; and the third portion and the fourth portioninclude different conductive materials. In some embodiments, the secondportion and the fourth portion include different materials. In someembodiments, the first portion and the third portion include samematerials. In some embodiments, the second portion and the fourthportion have different sizes in the second direction.

In an embodiment, a method of fabricating a semiconductor deviceincludes: forming a fin structure that extends in a first direction;forming a dummy gate structure that extends in a second directiondifferent from the first direction; forming source/drain regions in thefin structure; and after the forming of the source/drain regions,replacing the dummy gate structure with metal-gate structure, whereinthe metal-gate structure includes a first portion formed over the finstructure and a second portion not formed over the fin structure, andwherein the first portion and the second portion include differentwork-function metals. In some embodiments, the forming of the dummy gatestructure includes forming a first dummy gate structure and a seconddummy gate structure that is spaced apart from the first dummy gatestructure in the first direction; the replacing of the dummy gatestructure is performed such that a first metal-gate structure and asecond metal-gate structure replace the first dummy gate structure andthe second dummy gate structure, respectively; the first metal-gatestructure includes the first portion and the second portion; the secondmetal-gate structure includes a third portion formed over the finstructure and a fourth portion not formed over the fin structure; andthe third portion and the fourth portion include different work-functionmetals. In some embodiments, the replacing of the dummy gate structureis performed such that: the second portion is separated from the finstructure by a first distance measured in the second direction; thefourth portion is separated from the fin structure by a second distancemeasured in the second direction; and the second distance is greaterthan or less than the first distance. In some embodiments, the replacingof the dummy gate structure is performed such that: the second portionhas a first dimension measured in the second direction; the fourthportion has a second dimension measured in the second direction; and thesecond dimension is greater than or less than the first dimension.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure. For example, by implementing different thicknessesfor the bit line conductor and word line conductor, one can achievedifferent resistances for the conductors. However, other techniques tovary the resistances of the metal conductors may also be utilized aswell.

What is claimed is:
 1. A device, comprising: a first gate structure thatincludes a first work function (WF) metal portion having a firstmaterial composition and a second WF metal portion having a secondmaterial composition different from the first material composition; anda second gate structure that includes a third WF metal portion having athird material composition and a fourth WF metal portion having a fourthmaterial composition different from the third material composition;wherein the second WF metal portion and the fourth WF metal portion havedifferent sizes.
 2. The device of claim 1, further comprising an activeregion, wherein the first WF metal portion and the third WF metalportion are located over the active region, but the second WF metalportion and the fourth WF metal portion are not located over the activeregion.
 3. The device of claim 2, wherein: the active region extends ina first direction in a top view; and the first gate structure and thesecond gate structure each extend in a second direction different fromthe first direction in the top view.
 4. The device of claim 3, whereinthe second WF metal portion is longer than the fourth WF metal portionin the second direction.
 5. The device of claim 4, wherein the second WFmetal portion is longer than the fourth WF metal portion in the seconddirection by at least 10%.
 6. The device of claim 2, wherein: the firstWF metal portion and the second WF metal portion form a first interface;the first interface is spaced apart from a boundary of the active regionby a first distance; the third WF metal portion and the fourth WF metalportion form a second interface; the second interface is spaced apartfrom the boundary of the active region by a second distance; and thesecond distance is greater than the second distance.
 7. The device ofclaim 6, wherein the second distance is at least 10% greater than thefirst distance.
 8. The device of claim 1, wherein: the first materialcomposition is identical to the third material composition; and thesecond material composition is identical to the fourth materialcomposition.
 9. The device of claim 1, wherein: the first materialcomposition is identical to the third material composition; and thesecond material composition is different from the fourth materialcomposition.
 10. The device of claim 1, further comprising: a third gatestructure that includes a fifth WF metal portion having the firstmaterial composition or third material composition and a sixth WF metalportion having the second material composition or the fourth materialcomposition, wherein the sixth WF metal portion has a different sizethan the second WF metal portion and the fourth WF metal portion. 11.The device of claim 1, wherein the first gate structure and the secondgate structure are associated with different threshold voltage values.12. A device, comprising: an active region that extends in a firstdirection; a first work function (WF) metal segment disposed over theactive region; a second WF metal segment disposed outside the activeregion, wherein the second WF metal segment is contiguous with the firstWF metal segment and has a different material composition than the firstWF metal segment; a third work function metal segment disposed over theactive region, wherein the third WF metal segment is spaced apart fromthe first WF metal segment in the first direction; and a fourth WF metalsegment disposed outside the active region, wherein the fourth WF metalsegment is contiguous with the third WF metal segment and has adifferent material composition than the third WF metal segment, andwherein the fourth WF metal segment is spaced apart from the second WFmetal segment in the first direction, and wherein the fourth WF metalsegment and the second WF metal segment have different dimensions in asecond direction different from the first direction.
 13. The device ofclaim 12, wherein the second WF metal segment and the fourth WF metalsegment have a same material composition.
 14. The device of claim 12,wherein a first boundary between the first WF metal segment and thesecond WF metal segment is located closer to the active region in thesecond direction than a second boundary between the third WF metalsegment and the fourth WF metal segment.
 15. The device of claim 12,further comprising: a fifth WF metal segment disposed over the activeregion; a sixth WF metal segment disposed outside the active region;wherein: the sixth WF metal segment is contiguous with the fifth WFmetal segment and has a different material composition than the fifth WFmetal segment and than the second or third WF metal segments; and thefifth WF metal segment has a same material composition as the first WFmetal segment.
 16. A method, comprising: forming an active region thatextends in a first direction; forming a first dummy gate and a seconddummy gate that each extend in a second direction different from thefirst direction; replacing the first dummy gate with a first functionalgate that includes a first work function (WF) metal layer, wherein afirst segment of the first WF metal layer is formed directly over theactive region, and wherein a second segment of the first WF metal layeris formed outside of the active region; and replacing the second dummygate with a second functional gate that includes a second WF metallayer, wherein a third segment of the second WF metal layer is formeddirectly over the active region, and wherein a fourth segment of thesecond WF metal layer is formed outside of the active region; wherein:the first segment and the second segment of the first WF metal layer areformed to have different material compositions; the third segment andthe fourth segment of the second WF metal layer are formed to havedifferent material compositions; and the second segment of the first WFmetal layer is formed to have a smaller dimension in the seconddirection than the fourth segment of the second WF metal layer.
 17. Themethod of claim 16, wherein the replacing the first dummy gate and thereplacing the second dummy gate are performed simultaneously.
 18. Themethod of claim 16, wherein the replacing the first dummy gate and thereplacing the second dummy gate are performed such that the firstsegment and the third segment each have a first WF metal composition,and that the third segment and the fourth segment each have a second WFmetal composition.
 19. The method of claim 16, wherein the replacing thefirst dummy gate and the replacing the second dummy gate are performedsuch that the fourth segment is further spaced apart from the activeregion than the second segment.
 20. The method of claim 16, furthercomprising: forming a dielectric isolation structure adjacent to theactive region, wherein the second segment and the fourth segment areeach formed directly over the dielectric isolation structure.